The present invention relates generally to communication systems, and particularly to communication systems where the same audio signal is to be simultaneously broadcasted from a plurality of remotely located transmitter stations or sites.
Simulcasting is used herein refers to the simultaneous broadcasting of the same audio signal from a plurality of radio transmitter or base stations. While simulcasting itself is not a new technique, previous simulcast communication systems have usually employed privately-owned microwave radio links between each of the base stations and a dispatcher station from which the audio signal is transmitted to the base stations for broadcasting over the air by radio waves. This is because the use of privately-owned microwave links readily allow for stable audio signal time delay and amplitude transmission characteristics without which the radio broadcast of the audio signal from the base stations would be distorted and unintelligible.
With several base stations located remotely from the dispatch station and from each other, the audio signal must necessarily be sent from the dispatch center to the base stations along separate transmission lines or links. If these transmission lines or links have different audio signal transmission characteristics, then the audio signal broadcasted from at least one of the base stations will be distorted with respect to the audio signals broadcasted from the other base stations. For example, one of the transmission lines or links employed may attenuate or otherwise distort portions of the audio signal at certain frequencies, while another of the transmission lines or links employed may attenuate or otherwise distort other portions of the audio signal at different frequencies. Thus, the "amplitude" transmission characteristics of one transmission line or link may not necessarily be the same as the "amplitude" transmission characteristics of another transmission line or link, even if the same type of transmission line or link is employed, and these may change with time.
Additionally, in the typical communication system the base stations will not be located equidistantly from the dispatch center, thereby causing transmission lines or links of different lengths or distances to be employed. These different distances over which the audio signal is transmitted necessarily causes the base stations to receive the audio signal at slightly different times. These time differences will in general cause the base stations to broadcast the audio signal out of time or phase with respect to each other. Accordingly, due to the various distances involved, the "time delay" transmission characteristics of one transmission line or link may be different from the "time delay" transmission characteristics of another transmission line or link, even if the same type of transmission line or link is employed, and these may also change with time.
While it may be possible to have the base stations located sufficiently far apart from each other or the power of their transmitters adjusted such that the audio signal can only be received from one base station at a time, in practice there is generally no well-defined boundary beyond which the audio signal cannot be received from a particular base station. Accordingly, there will be overlapping areas of reception which will depend at least in part upon the local terrain and atmospheric conditions, thereby making these overlapping areas unpredictable from a design standpoint.
Particularly with respect to FM communication systems, where the audio signal is frequency modulated before being broadcasted from the base stations, it is essential that the transmission lines or links employed have equalized transmission characteristics, because the demodulated sum of the two frequency modulated audio signals will not be a linear sum of their audio modulations. Accordingly, in the overlapping areas of reception the output of a receiver tuned to the carrier frequency may be severely distored or unintelligible when transmission lines or links with different amplitude and/or time delay transmission characteristics are employed.
This situation is further exacerbated in mobile radio communication systems which employ a continuous tone-encoded subaudible squelch (CTCSS) signal to enable the receiver contained in one or more of the mobile stations to demodulate the frequency modulated audio signal. If these squelch signals are not synchronized such that they are broadcasted from each of the base stations with the same amplitude, frequency and phase, then in the overlapping areas of reception the receive in a mobile station will demodulate the distortion products of the CTCSS, thereby interfering with desired audio signal.
While microwave links may be employed to obviate the above-identified problems, the high cost and long lead time associated with the microwave system installations necessary to satisfy the technical requirement of simulcasting have inhibited their widespread use. Accordingly, a common solution to these problems is to place the dispatcher behind a radio console from which base stations are selected one at a time to broadcast the frequency modulated audio signal. However, this solution is unnecessarily time consuming and also inhibits mobile stations in one area from monitoring the communications of the mobile stations in another area. A further discussion of the problems involved with simulcasting may be found in a paper entitled "Automatic Equalization for Simulcasting" presented at the May 23, 1982 meeting of the Institute of Electrical and Electronic Engineering Professional Society's Vehicular Technology Group by the applicant of the present invention. This paper is hereby incorporated by reference.
Accordingly, it is a principal object of the present invention to provide a simulcast communication system in which an audio signal may be simultaneously broadcasted from a plurality of base stations which are located remotely from a dispatch station used to transmit the audio signal to the base stations along conventional voice grade telephone lines.
It is a more specific object of the present invention to provide a simulcast communication system which automatically equalizes the audio signal transmission characteristics from the dispatch station to each of the base stations.
It is a further object of the present invention to provide a simulcast communication system which automatically synchronizes the continuous tone-coded subaudible squelch signal generated at each of the base stations.
It is another object of the present invention to provide the automatic equalization circuitry required to convert a conventional FM communication system into a simulcast communication system.
To achive the foregoing objects, the present invention provides first circuit means for equalizing the audio signal transmission characteristics from the dispatch station to each of the base stations in response to at least one test signal broadcasted from each of the base stations, and second circuit means for generating a synchronized squelch signal at each of the base stations in response to a pilot signal transmitted to each of the base stations with the audio signal and a phasing signal transmitted to each of the base stations at the beginning of the audio signal. The first circuit means includes test signal means for generating the test signals. These test signals are transmitted from the dispatch station to the base stations in a predetermined sequence, such that one or more of these test signals are broadcasted from only one base station at a time. The first circuit means also includes digital processing means for determining the audio signal transmission characteristics required to equalize each of the base stations from test response signals which each represent a test signal that has been broadcasted from one of the base stations. The first circuit means further includes programmable equalization means for equalizing the audio signal transmission characteristics from the dispatch station to each of the base stations in response to the digital processing means.
The second circuit means generally includes pilot signal means for generating the pilot signal, phasing signal means for generating the phasing signal, and detection means associated with each of the base stations for detecting the pilot signal and the occurrence of the phasing signal. The second circuit means also includes synchronization means associated with each of the base stations and responsive to the detected pilot and phasing signals, for generating the synchronized squelch signal at each of the base stations. Specifically, this synchronized squelch signal is phase locked to the occurrence of the phasing signal.
The present invention also provides a method of equalizing the audio signal transmission characteristics from the dispatch station to each of the base stations in a communication system where the audio signal is transmitted from the dispatch station to each of the base stations along telephone lines. The present invention further provides a method of generating synchronized squelch signal to be simultaneously broadcasted from a plurality of base stations in a communication system where the audio signal is transmitted from the dispatch station to each of the base stations along telephone lines.